What was different? What allowed US forces to resist the siege, whereas French forces eventually were overwhelmed? Was it just the technological difference between time periods? Were the Vietnamese trying to achieve the same type of victory at Khe Sahn as they achieved at Dien Bien Phu? Why weren't they as successful?
The two situations were very different in terms of technology, how we used that technology, level of surprise in the surprise attack, and the scale of the two battles.
At Dien Bien Phu, the French were unaware that the Vietnamese had mobile heavy artillery. That revelation probably was a shock for the French when they were unprepared for heavy shelling, and the French artillery commander was so distraught by this unpreparedness that he committed suicide early enough in the battle for it to have an effect on the French soldiers. Superior Vietnamese artillery positioning also had a great effect.
At Khe Sanh, the Americans had one distinct advantage: the helicopter. The mass helicopter resupply was on such a bigger scale than Dien Bien Phi, where the French only lost 28 aircraft from the air due to flak, and only 2 helicopters. Khe Sanh had a much larger Air Force presence, and the Americans were therefore able to resupply their soldiers much better in my opinion. Helicopters had become a massive part of the United States's strategy after the battles of Ap Bac and Ia Drang. The United States also dropped 100,000 tons of bombs at Khe Sanh during Operation Niagra, just adding to the massive Air Force that was launched to support the ground troops. Operation Pegasus, the ground relief force broke through at Khe Sanh also helped a ton as well.
The success of the Vietnamese against these two forces is not as different as one might think. The French were outright defeated at Dien Bien Phu, whereas at Khe Sanh the American tactic of fighting a "war of attrition" was partially achieved. However, the Americans left Khe Sanh, destroying anything of importance there. It's pretty easy to see why both sides claimed victory, the Americans killed a bunch of people and the Vietnamese forced a retreat of sorts.
TL;DR: helicopter tactics, amount of Air Force, France underestimating the cunning of Vo Nguyen Giap and the Viet Cong. Khe Sanh: different objectives.
People's Army, People's War - Vo Nguyen Giap. (If you are looking for an unreal source on the Vietnam and French Indochina Wars, it doesn't get much better than Ho Chi Minh's general's memoirs/journal/battle plans.)
Close Air Support and the Battle for Khe Sanh - marine corps collaboration
Hope this helps, threw it together on mobile so I don't know if it's going to flow as well as opposed to writing it on a computer
e: I'd love to know where the downvotes came from. If you believe I have incorrect information on the subject, please tell me so I don't perpetuate incorrect history.